Java的@Repeatable注解使用詳細解析

更新時間：2024年02月02日 08:32:51 作者：小徐也要努力鴨

這篇文章主要介紹了Java的@Repeatable注解使用詳細解析,java8新增了注解@Repeatable,在hibernate-validator的源碼注解如@MAX、@NotNull等中,有@Repeatable注解的使用,需要的朋友可以參考下

1 前言

java8新增了注解@Repeatable，在hibernate-validator的源碼注解如@MAX、@NotNull等中，有@Repeatable注解的使用，源碼示例如下：

/*
 * Jakarta Bean Validation API
 *
 * License: Apache License, Version 2.0
 * See the license.txt file in the root directory or <http://www.apache.org/licenses/LICENSE-2.0>.
 */
package javax.validation.constraints;
import static java.lang.annotation.ElementType.ANNOTATION_TYPE;
import static java.lang.annotation.ElementType.CONSTRUCTOR;
import static java.lang.annotation.ElementType.FIELD;
import static java.lang.annotation.ElementType.METHOD;
import static java.lang.annotation.ElementType.PARAMETER;
import static java.lang.annotation.ElementType.TYPE_USE;
import static java.lang.annotation.RetentionPolicy.RUNTIME;
import java.lang.annotation.Documented;
import java.lang.annotation.Repeatable;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;
import javax.validation.Constraint;
import javax.validation.Payload;
import javax.validation.constraints.NotNull.List;
/**
 * The annotated element must not be {@code null}.
 * Accepts any type.
 *
 * @author Emmanuel Bernard
 */
@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER, TYPE_USE })
@Retention(RUNTIME)
@Repeatable(List.class)
@Documented
@Constraint(validatedBy = { })
public @interface NotNull {
	String message() default "{javax.validation.constraints.NotNull.message}";
	Class<?>[] groups() default { };
	Class<? extends Payload>[] payload() default { };
	/**
	 * Defines several {@link NotNull} annotations on the same element.
	 *
	 * @see javax.validation.constraints.NotNull
	 */
	@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER, TYPE_USE })
	@Retention(RUNTIME)
	@Documented
	@interface List {
		NotNull[] value();
	}
}

2 使用

參考hibernate-validator注解，使用@Repeatable：

2.1 新建注解RepeaDemo

暫時不添加@Repeatable：

package com.xiaoxu.tool.demo;

import java.lang.annotation.*;

@Target(value = {ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RepeaDemo {
    String id();
    String name();
}

新建demo類：

package com.xiaoxu.tool.demo;
import java.lang.reflect.Field;
import java.util.Arrays;
/**
 * @author xiaoxu
 * @date 2022-05-17
 * spring_boot:com.xiaoxu.tool.demo.TestRepeatable
 */
public class TestRepeatable {
    @RepeaDemo(id = "1001",name = "荔枝")
    private String fruit1;
    @RepeaDemo(id = "1002",name = "葡萄")
    private String fruit2;
    public static void demo1(){
        Class<?> a = TestRepeatable.class;
        Field[] fruits = a.getDeclaredFields();
        Arrays.stream(fruits).forEach(f->{
            if(f.isAnnotationPresent(RepeaDemo.class)){
                RepeaDemo anno = f.getAnnotation(RepeaDemo.class);
                String id = anno.id();
                String name = anno.name();
                System.out.printf("水果id:%s,水果名稱:%s%n",id,name);
            }
        });
    }
    public static void main(String[] args) {
        demo1();
    }
}

執(zhí)行結(jié)果如下：

水果id:1001,水果名稱:荔枝
水果id:1002,水果名稱:葡萄

但是問題是，如果希望在字段上增加多個注解，那么會提示Duplicate annotation.錯誤：

在這里插入圖片描述

于是考慮新增一個RepeaDemos注解，用于存儲@RepeaDemo注解數(shù)組，就可以達到多個注解的使用了，如下：

package com.xiaoxu.tool.demo;

import java.lang.annotation.*;

@Target(value = {ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RepeaDemos {
    RepeaDemo[] repeaDemos();
}

修改TestRepeatable代碼：

package com.xiaoxu.tool.demo;

import java.lang.reflect.Field;
import java.util.Arrays;

/**
 * @author xiaoxu
 * @date 2022-05-17
 * spring_boot:com.xiaoxu.tool.demo.TestRepeatable
 */
public class TestRepeatable {
    @RepeaDemos(repeaDemos = {
        @RepeaDemo(id = "1001",name = "荔枝"),
        @RepeaDemo(id = "1003",name = "沃柑")
    })
    private String fruit1;

    @RepeaDemo(id = "1002",name = "葡萄")
    private String fruit2;

    public static void demo1(){
        Class<?> a = TestRepeatable.class;
        Field[] fruits = a.getDeclaredFields();
        Arrays.stream(fruits).forEach(f->{
            if(f.isAnnotationPresent(RepeaDemo.class)){
                RepeaDemo anno = f.getAnnotation(RepeaDemo.class);
                String id = anno.id();
                String name = anno.name();
                System.out.printf("水果id:%s,水果名稱:%s%n",id,name);
            }
        });
    }

    public static void demo2(){
        Class<?> a = TestRepeatable.class;
        Field[] fruits = a.getDeclaredFields();
        Arrays.stream(fruits).forEach(f->{
            if(f.isAnnotationPresent(RepeaDemos.class)){
                RepeaDemos anno = f.getAnnotation(RepeaDemos.class);
                RepeaDemo[] res= anno.repeaDemos();
                Arrays.stream(res).forEach(r-> System.out.printf("水果id:%s,水果名稱:%s%n",r.id(),r.name()));
            }
            if(f.isAnnotationPresent(RepeaDemo.class)){
                RepeaDemo anno = f.getAnnotation(RepeaDemo.class);
                String id = anno.id();
                String name = anno.name();
                System.out.printf("水果id:%s,水果名稱:%s%n",id,name);
            }
        });
    }
    public static void main(String[] args) {
        demo2();
    }
}

執(zhí)行結(jié)果如下：

水果id:1001,水果名稱:荔枝
水果id:1003,水果名稱:沃柑
水果id:1002,水果名稱:葡萄

2.2 @Repeatable注解使用

對于上述注解使用，@Repeatable注解的作用就是使得@RepeaDemo注解可以多個添加在字段上，而最外層無須包裹@RepeaDemos注解，當然，@RepeaDemos注解任然需要保留，修改如下：

修改@RepeaDemos注解（注意@RepeaDemos注解，必須改為value()才可以）：

package com.xiaoxu.tool.demo;

import java.lang.annotation.*;

@Target(value = {ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface RepeaDemos {
    RepeaDemo[] value();
}

修改@RepeaDemo注解，增加@Repeatable，參數(shù)為RepeaDemos.class：

package com.xiaoxu.tool.demo;

import java.lang.annotation.*;

@Target(value = {ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Repeatable(RepeaDemos.class)
@Documented
public @interface RepeaDemo {
    String id();
    String name();
}

修改demo類：

package com.xiaoxu.tool.demo;
import java.lang.reflect.Field;
import java.util.Arrays;
/**
 * @author xiaoxu
 * @date 2022-05-17
 * spring_boot:com.xiaoxu.tool.demo.TestRepeatable
 */
public class TestRepeatable {
//    @RepeaDemos(value = {
//        @RepeaDemo(id = "1001",name = "荔枝"),
//        @RepeaDemo(id = "1003",name = "沃柑")
//    })
    @RepeaDemo(id = "1001",name = "荔枝")
    @RepeaDemo(id = "1003",name = "沃柑")
    private String fruit1;
    @RepeaDemo(id = "1002",name = "葡萄")
    private String fruit2;
    public static void demo1(){
        Class<?> a = TestRepeatable.class;
        Field[] fruits = a.getDeclaredFields();
        Arrays.stream(fruits).forEach(f->{
            if(f.isAnnotationPresent(RepeaDemo.class)){
                RepeaDemo anno = f.getAnnotation(RepeaDemo.class);
                String id = anno.id();
                String name = anno.name();
                System.out.printf("水果id:%s,水果名稱:%s%n",id,name);
            }
        });
    }
    public static void demo2(){
        Class<?> a = TestRepeatable.class;
        Field[] fruits = a.getDeclaredFields();
        Arrays.stream(fruits).forEach(f->{
            if(f.isAnnotationPresent(RepeaDemos.class)){
                RepeaDemos anno = f.getAnnotation(RepeaDemos.class);
                RepeaDemo[] res= anno.value();
                Arrays.stream(res).forEach(r-> System.out.printf("水果id:%s,水果名稱:%s%n",r.id(),r.name()));
            }
            if(f.isAnnotationPresent(RepeaDemo.class)){
                RepeaDemo anno = f.getAnnotation(RepeaDemo.class);
                String id = anno.id();
                String name = anno.name();
                System.out.printf("水果id:%s,水果名稱:%s%n",id,name);
            }
        });
    }
    public static void main(String[] args) {
        demo2();
    }
}

執(zhí)行效果如下：

水果id:1001,水果名稱:荔枝
水果id:1003,水果名稱:沃柑
水果id:1002,水果名稱:葡萄

可見，如下兩種方式，獲取的結(jié)果是一致的，其實@Repeatable注解就是一種語法糖（類似于python的裝飾器語法糖等等），本質(zhì)上在java編譯時，使用如下的第2種注解方式，等同于使用第1種：

//1：
@RepeaDemos(value = {
    @RepeaDemo(id = "1001",name = "荔枝"),
    @RepeaDemo(id = "1003",name = "沃柑")
})
//2:
@RepeaDemo(id = "1001",name = "荔枝")
@RepeaDemo(id = "1003",name = "沃柑")